This invention relates to printing sleeves which are readily mountable onto and dismountable from printing cylinders, and more particularly to printing sleeves which are expandably mountable and dismountable employing a pressurized gas.
In past printing operations, flexible printing plates were mounted onto the outer surface of a printing cylinder. These plates were used for printing of ink images onto a printing medium. Typically, the back of the plates was adhered directly to the printing cylinder. Since these plates were not readily interchangable from one cylinder to another, the use of a multiplicity of printing cylinders to perform a multiplicity of jobs was required. This presented severe storage and cost problems to the end user.
Therefore, in an effort to overcome this problem, printing sleeves were developed which were mountable onto and dismountable from the printing cylinders. Compressed gas, generally compressed air, passing in a substantially radial direction from holes located within the printing cylinders, was used to expand the sleeve to a limited extent for facilitating the mounting and dismounting operations.
The first patent to describe this latter mode of mounting and dismounting of a printing sleeve was U.S. Pat. No. 3,146,709. In that patent, a "wound" printing sleeve, i.e., a helically wound paper sleeve, was fitted onto a hollow printing sleeve. The printing sleeve was used as a carrier roll for rubber printing plates attached thereto. Air pressure was radially applied through the holes in the external surface of the printing cylinder for limited expansion of the sleeve. The sleeve was then axially mounted onto the printing cylinder by moving the cylinder to an upright position and filling the internal chamber of the cylinder with compressed air. As the sleeve was moved over the upper end of the cylinder, the exiting air expanded the sleeve and a lubricating air film was interposed between the inner sleeve and the outer cylinder. This air film permitted the axial movement of the sleeve to a position about the cylinder. When the sleeve was in such a position, the air flow was terminated, and the sleeve contracted in place about the cylinder.
However, difficulty has been encountered when wound sleeves are employed since expansion does not effectively take place unless high pressure air, substantially higher than the 50-100 psi air generally available in production facilities, is radially conveyed between the sleeve and the printing cylinder to facilitate the mounting and dismounting operation. This expandability problem occurs because of the thickness of the sleeve walls and the nature of the materials of construction. If pressures above the available air pressure at the production facility are required to expand the sleeve, auxiliary sources of compressed air must be purchased. For example, in printing operations where sleeve thicknesses of about 0.015" or greater are required, such as in the process printing industry, wound sleeves cannot readily be employed because they do not undergo the requisite expansion using available production compressed air. Furthermore, these wound sleeves cannot be effectively used because of the leakage problems inherent in their design, which in this case, U.S. Pat. No. 3,146,709, comprises a polyester film held in position by helically-wound paper tape. This type of construction forms a leakage path for the air and reduces the effectiveness of the lubricating fluid.
In order to overcome the problems inherent in the U.S. Pat. No. 3,146,709 wound printing sleeve, U.S. Pat. No. 3,978,254 has provided a mechanically adhered wound printing sleeve in which three layers of adhesive tape are helically wound about a mandrel to form a carrier sleeve, with two of the helixes being wound at the same angle and the remaining helix being wound at a different angle. The convolution of the helixes are said to impart some degree of strength, rigidity and leakage protection to the printing sleeve. Neither of the printing sleeves of U.S. Pat. No. 3,146,709 or U.S. Pat. No. 3,978,254 is unitary in construction, but is instead fabricated of a composite of wound materials. Furthermore, the outer surface of the U.S. Pat. No. 3,978,254 wound sleeve has a plurality of surface irregularities formed therein and is therefore not "round" to the extent required by the flexographic printing industry. These carrier sleeves are made of a flexible, thin tape material which provides a minimum of structural integrity which exhibit minimal strength and durability properties. Moreover, as the printing plates are adhered to the printing sleeve they are moved from one position to another as they are aligned on the plate surface. In order to trim excess material from the plate from the sleeve surface, they must be cut with a sharp instrument such as a knife. The synthetic plastic tape used to form the above-described sleeve cannot withstand even the minor cutting action required in positioning of the printing plates.
Another type of printing sleeve is one which is made of a metallic material. As in the case of wound sleeves, metallic sleeves are not readily expandable and therefore must have a wall thickness which is be quite thin, i.e., thicknesses of up to only about 0.005", in order to be capable of undergoing the limited expansion required of printing sleeves. As indicated above, this minimum thickness level required of metallic sleeves is a problem in applications such as process printing and the like. Moreover, printing metallic sleeves are not durable and are readily damages. For instance, they can easily form kinks in their outer surface when they are stored without being supported by a printing cylinder.
Dimensional stability is a problem in printing applications requiring that the outer surface of a printing sleeve structure have a true cylindrical shape. In some cases, this true cylindrical shape must even be within a 0.001"-0.0025" tolerance level in order to be acceptable in, for example, uses such as in the process printing industry. The outer printing surface in these applications must accurately conform to a uniformly constant, cylindrical outer shape in order to accurately imprint a print image onto a printing medium. Many of these prior art printing sleeves do not meet these requisite tolerance levels.
U.S. Pat. No. 4,144,812 and U.S. Pat. No. 4,144,813 provide noncylindrical printing sleeves and associated air-assisted printing rolls designed in a tapered or stepped-transition configuration, the change in the sleeve or printing cylinder diameter from one end to the other being progressive, i.e., increasing or decreasing according to the direction one is moving along the printing sleeve or roll. The printing roll comprises an outer surface having one end of a diameter greater than the other longitudinal end. The printing sleeve has an inner surface designed to form an interference fit with the outer surface of the printing roll only at the designated working position, and not along the entire axial uniform cross-sectional extent of the tapered sleeve.
This non-cylindrical sleeve is fabricated of a highly rigid material having a low degree of expandability. These sleeves have a thickness of about 0.015". An extremely high air pressure, in excess of 125 psi, and typically about 250 psi or higher, is thus required to be introduced as the sleeve is being fitted onto the underlying air-assisted, printing roll in order to extend the radial dimension of the printing sleeve to a position capable of achieving complete coverage of the printing cylinder by the sleeve. Complete coverage is required in this system to achieve a proper interference fit. Since a pressure in excess of 125 psi is required herein, the system must satisfy various governmental regulations relating to pressure-rated containers. Conventional cylindrically-shaped, air-assisted printing presently on hand cannot readily be retrofitted to accommodate this non-cylindrical configuration because they cannot meet the abovedescribed pressure-rating requirement. Therefore, they must be replaced, at great cost, by new non-cylindrical printing cylinders capable of meeting these government regulations.
U.S. Pat. No. 4,119,032, describes an air-assisted printing cylinder mounted in a printing machine in such a way that a printing sleeve on its outer surface can be removed axially while the roll remains substantially in its working position. One end bearing of the printing cylinder is removably secured to a side of the machine frame. For axial positioning, an adjustable restrainer engages the roll axle at that end. Beyond the other side frame a counterpoise acts on the printing cylinder axle to support the printing cylinder when one end bearing is removed.
Finally, in U.S. Pat. No. 4,089,265, a flexographic printing roll is provided comprising a rigid base tube having perforations in the form of a plurality of small apertures and a printing sleeve on the tube strained to grip the tube to retain the sleeve securely on the tube. There is no underlying printing cylinder in the conventional sense in this system.
Therefore, a need exists for a cylindrically-shaped printing sleeve which is unitary and airtight, which can be frictionally mounted onto conventional cylindrically-shaped printing cylinders having a complementary outside diameter, which is readily expandable using a low pressure fluid, and which has a wall thickness and a true outer wall surface capable of being used in process printing applications.